1. Field of the Invention
This invention relates to microspheres for controlled release of a bioactive agent. In one form, the microspheres include poly(propylene fumarate), poly(lactic-co-glycolic acid)), and a bioactive agent. The microspheres may be covalently attached to a poly(propylene fumarate) scaffold for tissue regeneration applications in which the bioactive agent is controllably released from the scaffold.
2. Description of the Related Art
In the field of tissue engineering, biodegradable polymeric biomaterials can serve as a scaffold to provide mechanical support and a matrix for the ingrowth of new tissue. As new tissue forms on the scaffold, the biomaterial degrades until it is entirely dissolved. The degradation products are eliminated through the body's natural pathways, such as metabolic processes. One example of the use of such biomaterials is as a temporary bone replacement to replace or reconstruct all or a portion of a living bone. The bone tissue grows back into the pores of the polymeric implant and will gradually replace the entire implant as the polymeric implant itself is gradually degraded in the in vivo environment.
Poly(propylene fumarate) (PPF) is one polymer that has been employed as such a biomaterial. Poly(propylene fumarate) is an unsaturated linear polyester that degrades in the presence of water into propylene glycol and fumaric acid, degradation products that are easily cleared from the human body by normal metabolic processes. Because the fumarate double bonds in poly(propylene fumarate) are reactive and crosslink at low temperatures, poly(propylene fumarate) can be an effective in situ polymerizable biomaterial. The high mechanical strength of cured poly(propylene fumarate) matrices and their ability to be crosslinked in situ makes them especially suitable for orthopedic applications.
Several poly(propylene fumarate) formulation methods and tissue regeneration applications for poly(propylene fumarate) have been developed. U.S. Pat. Nos. 6,423,790, 6,384,105, 6,355,755, 6,306,821, 6,124,373 and 5,733,951 describe various synthesis methods and applications for poly(propylene fumarate). The disclosure of these patents and all other patents and publications mentioned herein are incorporated herein by reference. An injectable, in situ polymerizable, biodegradable poly(propylene fumarate) scaffold for bone regeneration has also been developed. See, Ishaug et al. “Bone formation by three-dimensional stromal osteoblast culture in biodegradable polymer scaffolds.” J Biomed Mater Res, 36(1): 17-28, 1997; and Ishaug, et al., “Osteoblast function on synthetic biodegradable polymers.”, J Biomed Mater Res, 28(12): 1445-53, 1994. The poly(propylene fumarate) scaffold can be shaped into the desired structure either in an ex vivo mold or in situ. Solid poly(propylene fumarate) hardens within 5-15 minutes to attain mechanical properties similar to cancellous bone (see, R. G. Payne, “Development of an injectable, in situ crosslinkable, degradable polymeric carrier for osteogenic cell populations.” Ph.D. thesis, Rice University, Houston, Tex., USA, 2001).
Controlled release of bioactive molecules such as cytokines and growth factors has also become an important aspect of tissue engineering because it allows modulation of cellular function and tissue formation at the afflicted site (see, Agrawal et al., “Biodegradable polymeric scaffolds for musculoskeletal tissue engineering.”, J Biomed Mater Res, 55(2): 141-50, 2001). The encapsulation of drugs, proteins and other bioactive molecules within degradable materials has long been known to be an effective way to control the release profile of the contained substance. More recently, microencapsulation has been found to have similar effects, and has been used for the controlled delivery of various drugs and active proteins.
A variety of methods is known by which compounds can be encapsulated in the form of microparticles. See, for example, U.S. Pat. Nos. 5,019,400 and 4,954,298 and European patent application EP 442671. In these methods, the material to be encapsulated (drugs or other active agents) is generally dissolved, dispersed, or emulsified, using stirrers, agitators, or other dynamic mixing techniques, in a solvent containing the polymeric material. Solvent is then removed from the microparticles and thereafter the microparticle product is obtained. Encapsulation techniques have been successfully applied to encapsulate growth factors such as transforming growth factor-beta (TGF-b), recombinant bone morphogenetic protein (rhBMP-2) and basic fibroblast growth factor (bFGF) in poly(lactic-co-glycolic acid) (PLGA) microspheres (see, Lu et al. “Controlled release of transforming growth factor beta 1 from biodegradable polymer microparticles”, J Biomed Mater Res, 50(3): 440-51, 2000; Lu et al., “TGF-beta 1 release from biodegradable polymer microparticles: its effects on marrow stromal osteoblast function”, J Bone Joint Surg Am, 83-A (Suppl 1 (Pt 2)): S82-91, 2001; Oldham et al., “Biological activity of rhBMP-2 released from PLGA microspheres”, J Biomech Eng, 122(3): 289-92, 2000; and Peter et al., “Effects of transforming growth factor beta 1 released from biodegradable polymer microparticles on marrow stromal osteoblasts cultured on poly(propylene fumarate) substrates.” J Biomed Mater Res, 50(3): 452-62, 2000).
The addition of growth factors within a poly(propylene fumarate) scaffold could result in a composite biomaterial that could serve both a structural role and a drug delivery role. For example, an injectable formulation consisting of poly(propylene fumarate) and growth factor-loaded microspheres could be used to fill bone defects. However, the most extensively studied bioabsorbable microsphere is made from poly(lactic-co-glycolic acid), and conventional poly(lactic-co-glycolic acid) microspheres would not be expected to attach to the wall of a poly(propylene fumarate) scaffold. As a result, the poly(lactic-co-glycolic acid) microspheres can migrate out of the scaffold. Due to the migration of the poly(lactic-co-glycolic acid) microspheres, the local sustained release of the protein is less controllable.
Therefore, there is a need for bioabsorbable microspheres that controllably release a bioactive agent and that can be attached to a scaffold for tissue regeneration such that the microspheres do not migrate out of the scaffold. Further, there is a need for a material that can be used to prepare a scaffold for tissue regeneration having attached microspheres that controllably release a bioactive agent. Also, there is a need for a scaffold for tissue regeneration that has attached microspheres which controllably release a bioactive agent and which do not migrate out of the scaffold.